Method for producing mechanically stable water-absorbent polymer particles

ABSTRACT

A process for producing water-absorbing polymer particles by polymerizing droplets of a monomer solution in a gas phase surrounding the droplets and postcrosslinking the polymer particles, wherein the postcrosslinked polymer particles are at least partly coated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase of International Application No.PCT/EP2007/063758, filed Dec. 12, 2007, which claims the benefit ofEuropean Patent Application No. 06126998.1, filed Dec. 22, 2006.

The present invention relates to a process for producing water-absorbingpolymerparticles by polymerizing droplets of a monomer solution in a gasphase surrounding the droplets and postcrosslinking the polymerparticles, wherein the postcrosslinked polymer particles are at leastpartly coated.

The preparation of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

Being products which absorb aqueous solutions, water-absorbing polymerare used to produce diapers, tampons, sanitary napkins and other hygienearticles, but also water-retaining agents in market gardening.

The properties of the water-absorbing polymers can be adjusted via thedegree of crosslinking. With increasing degree of crosslinking, the gelstrength rises and the absorption capacity falls.

To improve the use properties, for example saline flow conductivity in aswollen gel bed (SFC) in the diaper and absorption under pressure (AUP),water-absorbing polymer particles are generally postcrosslinked. As aresult, only the degree of crosslinking of the particle surface rises,as a result of which the absorption under pressure (AUP) and thecentrifuge retention capacity (CRC) can be at least partly decoupled.This postcrosslinking can be performed in the aqueous gel phase.However, dried, ground and screened-off polymer particles (base polymer)are preferably coated on the surface with a post crosslinker, thermallypostcrosslinked and dried. Crosslinkers suitable for this purpose arecompounds which comprise groups which can form covalent bonds with thecarboxylate groups of the hydrophilic polymer.

Spray polymerization allows the process steps of polymerization anddrying to be combined. In addition, the particle size can be set withincertain limits by virtue of suitable process control.

The production of water-absorbing polymer particles by polymerizingdroplets of a monomer solution is described, for example, in EP 348 180A1, WO 96/40427 A1, U.S. Pat. No. 5,269,980, DE 103 14 466 A1, DE 103 40253 A1, DE 10 2004 024 437 A1 and DE 10 2005 002 412 A1, and also theprior German application 102006001596.7.

DE 10 2004 042 946 A1, DE 10 2004 042 948 A1, DE 10 2004 042 955 A1 andDE 10 2005 019 398 A1 describe the production of thickeners by spraypolymerization.

WO 2006/079631 A1 describes a process for dropletization polymerization,wherein the polymer particles are dried in a fluidized bed andoptionally postcrosslinked.

EP 703 265 A1 describes a process for improving the attrition resistanceof water-absorbing polymer particles by coating with film-formingpolymers.

EP 755 964 A2 likewise discloses a process for improving the attritionresistance of water-absorbing polymer particles by coating the particleswith waxes.

It was an object of the present invention to provide an improved processfor producing water-absorbing polymer particles by polymerizing dropletsof a monomer solution in a gas phase surrounding the droplets.

The object was achieved by a process for producing water-absorbingpolymer particles by polymerizing droplets of a monomer solutioncomprising

a) at least one ethylenically unsaturated monomer,

b) optionally at least one crosslinker,

c) at least one initiator,

d) water,

in a gas phase surrounding the droplets, the resulting polymer particlesbeing postcrosslinked, which comprises at least partly coating thepostcrosslinked polymer particles.

Polymerization of monomer solution droplets in a gas phase surroundingthe droplets (“dropletization polymerization”) affords round polymerparticles of high mean sphericity (mSPHT). The mean sphericity is ameasure of the roundness of the polymer particles and can be determined,for example, with the Camsizer® image analysis system (Retsch TechnologyGmbH; Germany).

The present invention is based on the finding that the polymer particlesobtained by dropletization polymerization are hollow spheres and thatthe hollow spheres can be damaged by mechanical stress.

The coating according to the invention allows the mechanical stabilityof the hollow spheres to be increased significantly. Suitable coatingagents are, for example, additives which increase the elasticity of theparticle surface, for example by lowering the glass transitiontemperature, and/or mechanically solidify the particle surface.

The coating can be produced by coating the water-absorbing polymerparticles prepared by dropletization polymerization in a known manner bymixing in the coating agent in the desired weight ratio. This coatingtakes place preferably in mixers with moving mixing tools, such as screwmixers, paddle mixers, disk mixers, plowshare mixers and shovel mixers.Particular preference is given to vertical mixers, very particularpreference to plowshare mixers and shovel mixers. Suitable mixers are,for example, Lödige mixers, Bepex mixers, Nauta mixers, Processallmixers and Schugi mixers. Very particular preference is given tohigh-speed mixers, for example of the Schuggi-Flexomix or Turbolizertype. Very particular preference is also given to fluidized bed mixers.

The postcrosslinked water-absorbing polymer particles are preferablycoated with water, an aqueous solution, an alkanolamine, a polymerand/or a wax.

The amount of water or aqueous solution used in the inventive coating ispreferably from 1 to 30% by weight, more preferably from 2 to 20% byweight, most preferably from 3 to 15% by weight.

In a preferred embodiment in the case of coating with water or anaqueous solution, a surfactant is added as a deagglomerating assistant,for example sorbitan monoesters such as sorbitan monococoate andsorbitan monolaurate, or ethoxylated variants thereof. Further verysuitable deagglomerating assistants are the ethoxylated and alkoxylatedderivatives of 2-propylheptanol which are sold under the trademarksLutensol® XL and Lutensol® XP (BASF Aktiengesellschaft, Germany).

The amount of the deagglomerating assistant used, based on thewater-absorbing polymer particles, is, for example, from 0.01 to 1% byweight, preferably from 0.05 to 0.5% by weight, more preferably from 0.1to 0.2% by weight.

The amount of alkanolamine used in the inventive coating is preferablyfrom 0.5 to 30% by weight, more preferably from 1 to 20% by weight, mostpreferably from 2 to 15% by weight, and the alkanolamines can also beused as a solution in a suitable solvent, for example water.

Suitable alkanolamines are mono-, di- and trialkanolamines, such asethanolamine, diethanolamine or triethanolamine. Preference is given totertiary alkanolamines, such as triethanolamine, methyldiethanolamine,dimethylaminodiglycol, dimethylethanol-amine andN,N,N′,N′-tetra(hydroxyethyl)ethylenediamine. Particular preference isgiven to triethanolamine.

The amount of polymer and/or wax is preferably from 0.005 to 10% byweight, more preferably from 0.05 to 5% by weight, most preferably from0.1 to 2% by weight, based in each case on the polymer particles.

The polymer may also be applied to the particle surface by applying theprecursors of the polymer to the particle surface, which do not react togive the desired polymer until they are on the particle surface, forexample by reaction of polyols with polyepoxides.

The polymers, precursors thereof or waxes can be applied to the particlesurface as aqueous dispersions, emulsions and/or polymer suspensions.

The polymers, precursors thereof or waxes may also be used in the formof a solution in an organic solvent or a mixture of water and an organicwater-miscible solvent. These aqueous dispersions, emulsions andsuspensions may also comprise a proportion of organic, possiblywater-miscible solvents.

Suitable organic solvents are, for example, aliphatic and aromatichydrocarbons such as n-hexane, cyclohexane, toluene and xylene, alcoholssuch as methanol, ethanol, isopropanol, ethylene glycol, propyleneglycol, glycerol and polyethylene glycols with a mean molecular weightof from 200 to 10 000, ethers such as diethyl ether, esters such asethyl acetate and n-butyl acetate, and ketones such as acetone and2-butanone.

Suitable water-miscible organic solvents are, for example, aliphatic C₁-to C₄-alcohols, such as methanol, isopropanol, tert-butanol, ethyleneglycol, propylene glycol, glycerol and polyethylene glycols having amean molecular weight of from 200 to 10 000, ethers and ketones such asacetone and 2-butanone.

The polymers and/or waxes may also be metered in as a melt.

The polymers and/or waxes usable in the process according to theinvention are preferably unreactive, i.e. they have no reactive groupswhich react with the groups on the surface of the polymer particles.

Preferred polymers and/or waxes are also especially those which do nottend to adhere within the temperature range between 0° C. and 80° C.

Preferred polymers to be used in accordance with the invention for thecoating are homo- and copolymers of vinyl esters, especially vinylacetate homopolymers and vinyl acetate copolymers with ethylene,acrylates, maleic esters, vinylamides and/or other vinyl acylderivatives.

Preference is also given to homo- and copolymers of acrylic andmethacrylic esters, for example copolymers of methyl methacrylate andn-butyl acrylate or 2-ethylhexyl acrylate.

These copolymers based on vinyl, acrylic and methacrylic esters maycomprise, as further comonomers, for example, styrene, butadiene,vinylamides, olefinically unsaturated carboxylic acids and derivativesthereof, olefinically unsaturated sulfonic acids and derivativesthereof, vinylphosphonic acid and derivatives thereof or polyglycolesters of unsaturated acids.

Examples of vinylamides are in particular N-vinylformamide,N-vinyl-N-methylacetamide and N-vinylpyrrolidone.

Examples of olefinically unsaturated carboxylic acids are especiallyacrylic acid, methacrylic acid, itaconic acid and maleic acid, and alsotheir alkali metal, ammonium and amine salts. Examples of derivatives ofthese olefinically unsaturated carboxylic acids are especially amidessuch as (meth)acrylamide, N-tert-butyl(meth)acrylamide andN-isopropyl(meth)acrylamide, but also N-methylolamides or ethers ofN-methylolamides, monoamides and imides of aliphatic amines, and alsoacrylonitrile.

Examples of olefinically unsaturated sulfonic acids are the salts ofvinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid,styrenesulfonic acid, allyl- and methallyl-sulfonic acid, especiallytheir alkali metal, ammonium and amine salts.

Examples of derivatives of vinylphosphonic acid are especially the mono-and diesters of C₁- to C₁₈-alcohols, for example the methyl, propyl orstearyl esters. The vinyl-phosphonic acid itself is present inparticular in the form of the mono- or disalt, preference being given tothe alkali metal, ammonium and amine salts.

Polyglycol esters of unsaturated acids are in particular hydroxyethyl(meth)acrylate or esters of acrylic and methacrylic acid withpolyalkylene oxide compounds of the general formula

where

-   X is hydrogen or methyl,-   n is from 0 to 50 and-   R is an aliphatic, araliphatic or cycloaliphatic C₁ to C₂₄ radical,    for example nonylphenyl.

In a preferred embodiment of the present invention, film-formingpolymers are used for the coating. These are in particular polymerswhich have polymer films having a breaking resistance of from 0.5 to 25N/mm, preferably from 1 to 20 N/mm, more preferably from 2 to 15 N/mm,most preferably from 5 to 10 N/mm, and an elongation at break of from 10to 10 000%, preferably from 20 to 5000%, more preferably from 50 to2000%, most preferably from 500 to 1000%. The breaking resistance andthe elongation at break are determined to DIN EN ISO 527.

Preferred polymers to be used according to the invention for the coatingare also film-forming polymers based on

-   -   polyacetals, i.e. reaction products of polyvinyl alcohols with        aldehydes such as butyraldehyde,    -   polyurethanes, i.e. polymers obtainable by polyaddition from        dihydric and higher polyhydric alcohols and isocyanates, for        example prepared from polyester diols and/or polyether diols        and, for example, 2,4- or 2,6-tolylene diisocyanate,        4,4-methylene di(phenylisocyanate) or hexamethylene diisocyanate        (see Houben-Weyl, Methoden der Organischen Chemie [Methods of        organic chemistry], 4th edition, Volume E20/2, pages 1561 to        1721),    -   polyureas, i.e. polymers which are obtainable by polyaddition of        diamines and diisocyanates or by polycondensation of diamines        with carbon dioxide, phosgene, carboxylic esters (e.g. activated        diphenyl carbonates) or urea, or by reacting diisocyanates with        water (see Houben-Weyl, Methoden der Organischen Chemie, 4th        edition, Volume E20/2, pages 1721 to 1752), polysiloxanes, the        base polymer used being in particular linear        dimethyl-polysiloxane whose end groups may have different        modification (see “Chemie und Technologie des kalthärtenden        Siliconkautschuks” [Chemistry and technology of cold-curing        silicone rubber], pages 49 to 64 in SILICONE—Chemie und        Technologie, [Symposium on Apr. 28, 1989] VULKAN-VERLAG, Essen),    -   polyamides, preference being given to copolyamides (see Plaste        Kautsch., Volume 25, pages 440 to 444 (1978)), as find use, for        example, for the production of coatings,    -   polyesters, i.e. polymers which are prepared by ring-opening        polymerization of lactones or by polycondensation of        hydroxycarboxylic acids or of diols and dicarboxylic acid        derivatives (see Houben-Weyl, Methoden der Organischen Chemie,        4th edition, Volume E20/2, pages 1404 to 1429),    -   epoxy resins which can be prepared from polyepoxides by        polyaddition reactions with suitable hardeners or by        polymerization via epoxy groups (see Houben-Weyl, Methoden der        Organischen Chemie, 4th edition, Volume 14/2, pages 462 to 552        and Volume E20/2, pages 1891 to 1994 (for example reaction        products of bisphenol A with epichlorohydrin)) or based on    -   polycarbonates, as preparable easily by reacting diglycols or        bisphenols with phosgene or carbonic diesters in        polycondensation or transesterification reactions (see        Houben-Weyl, Methoden der Organischen Chemie, 4th edition,        Volume E 20/2, pages 1443 to 1457).

Particularly preferred polymers to be used in accordance with theinvention for the coating are homo- and copolymers of acrylic andmethacrylic esters, and also polymers based on polyacetals.

It is also possible to use mixtures of two or more of the abovementionedpolymers. The mixing ratios are completely uncritical and should beadjusted to the particular circumstances.

Preferred polymers to be used in the process according to the inventionare also elastic polymers. These are polymers with rubber-elasticbehavior which can be elongated at 23° C. repeatedly to at least twicetheir length and, after removal of the force required for theelongation, immediately reassume their approximate starting length.

Representative examples of suitable elastic polymers are natural andsynthetic latices which are typically used as binders and elastomericadhesives in the production of air-drying absorbing products. Inaddition to the latices described in the examples, it is taken intoaccount that any natural or synthetic elastomer suitable for theformation of a latex dispersion is suitable for use in the presentinvention. It is therefore possible to use natural rubber, polybutadienerubber, styrene-butadiene rubber, arylnitrile-butadiene rubber,poly-2-chlorobutadiene rubber, polyisoprene rubber, isoprene-isobutylenecopolymers, ethylene-propylene rubber, ethylene-vinyl acetatecopolymers, chlorinated polyethylene, chlorosulfonated polyethylene,acrylic rubber, ethylene-acrylate copolymers, epichlorohydrin rubber,polypropylene oxide rubber and polyurethanes.

In a further preferred embodiment of the present invention, waxes areapplied to the surface of the postcrosslinked water-absorbing polymerparticles.

According to the formulation of the Deutschen Gesellschaft fürFettwissenschaft (DGF) [German Society for Fat Science] of 1974 (seeDGF-Einheitsmethoden: Untersuchung von Fetten, Fettprodukten undverwandten Stoffen, Abteilung M: Wachse und Wachsprodukte [DGF standardmethods: analysis of fats, fat products and related substances, divisionM: waxes and wax products]; Wissenschaftliche Verlagsgesellschaft,Stuttgart, 1975), a wax is understood in particular to mean a substancewhich, irrespective of its chemical composition and its natural orsynthetic origin, is generally characterized by the followingphysicomechanical properties:

-   -   kneadable at 20° C., solid to brittle and hard;    -   coarsely to finely crystalline, transparent to opaque, but not        grasslike;    -   melting above 40° C. without decomposition;    -   being of comparatively low viscosity even a little above the        melting point and not stringing;    -   strongly temperature-dependent in consistency and solubility;    -   polishable under gentle pressure.

Preferred waxes are especially those whose melting and dropping pointsare within the temperature range between 30 and 180° C., more preferablybetween 40 and 180° C., most preferably between 40 and 170° C. Thedropping point is determined by DGF standard method DGF-M-III 3 (75)(Wissenschaftliche Verlagsgesellschaft, Stuttgart).

Waxes to be used in accordance with the invention are, for example,natural waxes, modified natural waxes, semisynthetic waxes and fullysynthetic waxes. Examples of natural waxes are recent waxes such asplant waxes or animal waxes. Examples of plant waxes are carnauba wax,candelilla wax, ouricury wax, sugarcane wax and retamo wax. Examples ofanimal waxes are insect waxes such as beeswax, ghedda wax and shellacwax, and also wool wax. Further examples of natural waxes are fossilwaxes such as mineral oil waxes or brown coal waxes and peat waxes.Examples of mineral oil waxes are ozokerite and tank bottom wax; anexample of a brown coal wax and peat wax is crude montan wax. Examplesof modified natural waxes are the waxes obtained by refining, such asthe macro- and microcrystalline paraffin waxes obtained from crude oildistillates or distillate residues, or chemically modified waxes such asdouble-bleached crude montan wax. Examples of semisynthetic waxes arethe acid waxes and ester waxes preparable from montan wax, the wax acidsproducible by paraffin oxidation, and also alcohol waxes and amidewaxes. Examples of fully synthetic waxes are hydrocarbon waxes such aspolyolefin waxes and Fischer-Tropsch waxes, and also synthetic waxeshaving oxygen-functional groups. Examples of synthetic waxes withoxygen-functional groups are acid waxes which are formed by oxidizingsynthetic hydrocarbon waxes or by copolymerizing or telomerizing olefinswith unsaturated carboxylic acids, ester waxes which are obtained byesterifying synthetic wax acids with synthetic alcohols and bycopolymerizing olefins with unsaturated esters such as vinyl acetate,alcohol waxes which are produced by an oxo process with subsequenthydrogenation and by hydrogenation of synthetic fatty acids, and alsoamide waxes which are obtained by reacting synthetic acids with amines.Examples of waxes which are obtained by oxidizing synthetic hydrocarbonwaxes are oxidates of polyethylene waxes.

Preferred waxes to be used in accordance with the invention are refined(i.e. deresinified and bleached) montan waxes, and also polyolefinwaxes.

Particularly preferred waxes to be used in accordance with the inventionare polyolefin waxes such as polyethylene waxes (high-pressurepolyethylene waxes, low-pressure polyethylene waxes, degradedpolyethylene waxes), oxidates of these polyethylene waxes, waxes basedon ethene-α-olefin copolymers, waxes based on ethylene-vinyl acetatecopolymers, waxes based on ethylene-styrene copolymers, waxes based onethylene-acrylic acid copolymers, and waxes based on wax mixtures ofpolyethylene waxes with poly(tetrafluoroethylene) waxes.

It is also possible to use mixtures of two or more of the abovementionedwaxes. The mixing ratios are completely uncritical and should beadjusted to the particular circumstances.

The monomer solutions to be used in the process according to theinvention to produce the water-absorbing polymer particles comprise atleast one ethylenically unsaturated monomer a), optionally at least onecrosslinker b), at least one initiator c) and water d).

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 50 g/100 g of water, and preferably have atleast one acid group each.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, maleic acid,fumaric acid and itaconic acid. Particularly preferred monomers areacrylic acid and methacrylic acid. Very particular preference is givento acrylic acid.

The preferred monomers a) have at least one acid group, the acid groupspreferably being at least partly neutralized.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The acid groups of the monomers a) are typically partly neutralized,preferably to an extent of from 25 to 85 mol %, preferentially to anextent of from 50 to 80 mol %, more preferably from 60 to 75 mol %, forwhich the customary neutralizing agents can be used, preferably alkalimetal hydroxides, alkali metal oxides, alkali metal carbonates or alkalimetal hydrogencarbonates, and mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Sodium and potassiumare particularly preferred as alkali metals, but very particularpreference is given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate, and mixtures thereof. Typically, the neutralizationis achieved by mixing in the neutralizing agent as an aqueous solution,as a melt or preferably also as a solid. For example, sodium hydroxidewith a water content significantly below 50% by weight may be present asa waxy material having a melting point above 23° C. In this case,metered addition as piece material or melt at elevated temperature ispossible.

The monomers a), especially acrylic acid, comprise preferably up to0.025% by weight of a hydroquinone monoether. Preferred hydroquinonemonoethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol is understood to mean compounds of the following formula

where R¹ is hydrogen or methyl, R² is hydrogen or methyl, R³ is hydrogenor methyl, and R⁴ is hydrogen or an acyl radical having from 1 to 20carbon atoms.

Preferred radicals for R⁴ are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically compatible carboxylic acids. The carboxylic acidsmay be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R¹═R²═R³=methyl, inparticular racemic alpha-tocopherol. R¹ is more preferably hydrogen oracetyl. RRR-alpha-tocopherol is especially preferred.

The monomer solution comprises preferably at most 130 ppm by weight,more preferably at most 70 ppm by weight, preferably at least 10 ppm byweight, more preferably at least 30 ppm by weight, in particular around50 ppm by weight, of hydroquinone monoether, based in each case onacrylic acid, acrylic acid salts also being considered as acrylic acid.For example, the monomer solution can be prepared by using acrylic acidhaving an appropriate content of hydroquinone monoether.

The polymerization inhibitors may also be removed from the monomersolution by absorption, for example on activated carbon.

Crosslinkers b) are compounds having at least two free-radicallypolymerizable groups which can be polymerized by a free-radicalmechanism into the polymer network. Suitable crosslinkers b) are, forexample, ethylene glycol dimethacrylate, diethylene glycol diacrylate,allyl methacrylate, trimethylolpropane triacrylate, triallylamine,tetraallyloxyethane, as described in EP 530 438 A1, di- andtriacrylates, as described in EP 547 847 A1, EP 559 476 A1, EP 632 068A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301A1 and in DE 103 31 450 A1, mixed acrylates which, as well as acrylategroups, comprise further ethylenically unsaturated groups, as describedin DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, asdescribed, for example, in DE 195 43 368 A1, DE 196 46 484 A1, WO90/15830 A1 and WO 2002/32962 A2.

Suitable crosslinkers b) are in particular N,N′-methylenebisacrylamideand N,N′-methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids of polyols, such as diacrylate or triacrylate, forexample butanediol diacrylate, butane-diol dimethacrylate, ethyleneglycol diacrylate or ethylene glycol dimethacrylate, and alsotrimethylolpropane triacrylate and allyl compounds such as allyl(meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters,tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allylesters of phosphoric acid and vinylphosphonic acid derivatives, asdescribed, for example, in EP 343 427 A2. Further suitable crosslinkersb) are pentaerythritol diallyl ether, pentaerythritol triallyl ether andpentaerythritol tetraallyl ether, polyethylene glycol diallyl ether,ethylene glycol diallyl ether, glycerol diallyl ether and glyceroltriallyl ether, polyallyl ethers based on sorbitol, and ethoxylatedvariants thereof. In the process according to the invention, it ispossible to use di(meth)acrylates of polyethylene glycols, thepolyethylene glycol used having a molecular weight between 100 and 1000.

However, particularly advantageous crosslinkers b) are di- andtriacrylates of 3- to 20-tuply ethoxylated glycerol, of 3- to 20-tuplyethoxylated trimethylolpropane, of 3- to 20-tuply ethoxylatedtrimethylolethane, in particular di- and triacrylates of 2- to 6-tuplyethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane,of 3-tuply propoxylated glycerol or of 3-tuply propoxylatedtrimethylolpropane, and also of 3-tuply mixed ethoxylated orpropoxylated glycerol or of 3-tuply mixed ethoxylated or propoxylatedtrimethylolpropane, of 15-tuply ethoxylated glycerol or of 15-tuplyethoxylated trimethylolpropane, and also of at least 40-tuplyethoxylated glycerol, of at least 40-tuply ethoxylated trimethylolethaneor of at least 40-tuply ethoxylated trimethylolpropane.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol.

The monomer solution comprises preferably at least 0.1% by weight,preferably at least 0.2% by weight, more preferably at least 0.3% byweight, most preferably at least 0.4% by weight, and preferably up to2.5% by weight, preferentially up to 2% by weight, more preferably up to1.5% by weight, most preferably up to 1% by weight, of crosslinker b),based in each case on monomer a).

The initiators c) used may be all compounds which disintegrate into freeradicals under the polymerization conditions, for example peroxides,hydroperoxides, hydrogen peroxide, persulfates, azo compounds and redoxinitiators. Preference is given to the use of water-soluble initiators.In some cases, it is advantageous to use mixtures of various initiators,for example mixtures of hydrogen peroxide and sodium or potassiumperoxodisulfate. Mixtures of hydrogen peroxide and sodiumperoxodisulfate can be used in any proportion.

Particularly preferred initiators c) are azo initiators such as2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride and2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, andphotoinitiators such as 2-hydroxy-2-methylpropiophenone and1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, redoxinitiators such as sodium persulfate/hydroxymethylsulfinic acid,ammonium peroxodisulfate/-hydroxymethylsulfinic acid, hydrogenperoxide/hydroxymethylsulfinic acid, sodium persulfate/ascorbic acid,ammonium peroxodisulfate/ascorbic acid and hydrogen peroxide/ascorbicacid, photoinitiators such as1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one, andmixtures thereof.

The initiators are used in customary amounts, for example in amounts offrom 0.001 to 5% by weight, preferably from 0.01 to 1% by weight, basedon the monomers a).

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Therefore, the monomer solution can be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingthrough with an inert gas, preferably nitrogen. The oxygen content ofthe monomer solution is preferably lowered before the polymerization toless than 1 ppm by weight, more preferably to less than 0.5 ppm byweight.

The monomer solution is dropletized for polymerization in the gas phase.

The solids content of the monomer solution is preferably at least 35% byweight, preferably at least 38% by weight, more preferably at least 40%by weight, most preferably at least 42% by weight. The solids content isthe sum of all constituents which are involatile after thepolymerization. These are monomer a), crosslinker b) and initiator c).

The oxygen content of the gas phase is preferably from 0.001 to 0.15% byvolume, more preferably from 0.002 to 0.1% by volume, most preferablyfrom 0.005 to 0.05% by volume.

As well as oxygen, the gas phase preferably comprises only inert gases,i.e. gases which, under reaction conditions, do not intervene in thepolymerization, for example nitrogen and/or steam.

The dropletization involves metering a monomer solution into the gasphase to form droplets. The dropletization of the monomer solution canbe carried out, for example, by means of a dropletizer plate.

A dropletizer plate is a plate having at least one bore, the liquidentering the bore from the top. The dropletizer plate or the liquid canbe oscillated, which generates a chain of ideally monodisperse dropletsat each bore on the underside of the dropletizer plate.

The number and size of the bores are selected according to the desiredcapacity and droplet size. The droplet diameter is typically 1.9 timesthe diameter of the bore. What is important here is that the liquid tobe dropletized does not pass through the bore too rapidly and thepressure drop over the bore is not too great. Otherwise, the liquid isnot dropletized, but rather the liquid jet is broken up (sprayed) owingto the high kinetic energy. The dropletizer is operated in the flowrange of laminar jet decomposition, i.e. the Reynolds number based onthe throughput per bore and the bore diameter is preferably less than2000, preferentially less than 1000, more preferably less than 500 andmost preferably less than 100. The pressure drop through the bore ispreferably less than 2.5 bar, more preferably less than 1.5 bar and mostpreferably less than 1 bar.

The dropletizer plate has typically at least one bore, preferably atleast 10, more preferably at least 50 and typically up to 10 000 bores,preferably up to 5000, more preferably up to 1000 bores, the borestypically being distributed uniformly over the dropletizer plate,preferably in so-called triangular pitch, i.e. three bores in each caseform the corners of an equilateral triangle.

The diameter of the bores is adjusted to the desired droplet size. Thedroplets generated have a mean droplet size of preferably at least 100μm, more preferably of at least 150 μm, most preferably of at least 200μm, the droplet diameter being determinable by light scattering.

It may be advantageous to place the dropletizer plate onto a carrierplate, in which case the carrier plate likewise has bores. In this case,the bores of the carrier plate have a greater diameter than the bores ofthe dropletizer plate and are arranged such that below each bore of thedropletizer plate is disposed a concentric bore of the carrier plate.This arrangement enables a rapid exchange of the dropletizer plate, forexample in order to generate droplets of another size.

However, the dropletization can also be carried out by means ofpneumatic drawing dies, rotation, cutting of a jet or rapidly actuablemicrovalve dies.

In a pneumatic drawing die, a liquid jet together with a gas stream isaccelerated through a diaphragm. The gas rate can be used to influencethe diameter of the liquid jet and hence the droplet diameter.

In the case of dropletization by rotation, the liquid passes through theorifices of a rotating disk. As a result of the centrifugal force actingon the liquid, droplets of defined size are torn off. Preferredapparatus for rotary dropletization are described, for example, in DE 4308 842 A1.

The emerging liquid jet can also be cut into defined segments by meansof a rotating blade. Each segment then forms a droplet.

In the case of use of microvalve dies, droplets with defined liquidvolumes are generated directly.

The gas phase preferably flows as carrier gas through the reactionchamber. The carrier gas can be conducted through the reaction chamberin cocurrent or in counter-current to the free-falling droplets of themonomer solution, preferably in cocurrent. After one pass, the carriergas is preferably recycled at least partly, preferably to an extent ofat least 50%, more preferably to an extent of at least 75%, into thereaction chamber as cycle gas. Typically, a portion of the carrier gasis discharged after each pass, preferably up to 10%, more preferably upto 3% and most preferably up to 1%.

The gas velocity is preferably adjusted such that the flow in thereactor is directed, for example no convection currents opposed to thegeneral flow direction are present, and is, for example, from 0.01 to 5m/s, preferably from 0.02 to 4 m/s, more preferably from 0.05 to 3 m/s,most preferably from 0.1 to 2 m/s.

The carrier gas is appropriately preheated to the reaction temperatureupstream of the reactor.

The reaction temperature in the thermally induced polymerization ispreferably from 70 to 250° C., more preferably from 100 to 220° C. andmost preferably from 120 to 200° C.

The reaction can be carried out under elevated pressure or under reducedpressure; preference is given to a reduced pressure of up to 100 mbarrelative to ambient pressure.

The reaction offgas, i.e. the carrier gas leaving the reaction chamber,may, for example, be cooled in a heat exchanger. This condenses waterand unconverted monomer a). The reaction offgas can then be reheated atleast partly and recycled into the reactor as cycle gas. A portion ofthe reaction offgas can be discharged and replaced by fresh carrier gas,in which case water and unconverted monomers a) present in the reactionoffgas can be removed and recycled.

Particular preference is given to a thermally integrated system, i.e. aportion of the waste heat in the cooling of the offgas is used to heatthe cycle gas.

The reactors can be trace-heated. In this case, the trace heating isadjusted such that the wall temperature is at least 5° C. above theinternal reactor temperature and condensation on the reactor walls isreliably prevented.

The reaction product can be withdrawn from the reactor in a customarymanner, preferably at the bottom by means of a conveying screw, and, ifappropriate, dried down to the desired residual moisture content and tothe desired residual monomer content.

The polymer particles are subsequently postcrosslinked for furtherimprovement of the properties. Suitable postcrosslinkers are compoundswhich comprise at least two groups which can form covalent bonds withthe carboxylate groups of the hydrogel. Suitable compounds are, forexample, alkoxysilyl compounds, polyaziridines, polyamines,polyamidoamines, di- or polyepoxides, as described in EP 83 022 A2, EP543 303 A1 and EP 937 736 A2, di- or polyfunctional alcohols asdescribed in DE 33 14 019 A1, DE 35 23 617 A1 and EP 450 922 A2, orβ-hydroxyalkylamides, as described in DE 102 04 938 A1 and U.S. Pat. No.6,239,230.

In addition, DE 40 20 780 C1 describes cyclic carbonates, DE 198 07 502A1 describes 2-oxazolidone and its derivatives such as2-hydroxyethyl-2-oxazolidone, DE 198 07 992 C1 describes bis- andpoly-2-oxazolidinones, DE 198 54 573 A1 describes2-oxotetrahydro-1,3-oxazine and its derivatives, DE 198 54 574 A1describes N-acyl-2-oxazolidones, DE 102 04 937 A1 describes cyclicureas, DE 103 34 584 A1 describes bicyclic amide acetals, EP 1199 327 A2describes oxetanes and cyclic ureas, and WO 2003/31482 A1 describesmorpholine-2,3-dione and its derivatives, as suitable postcrosslinkers.

It is also possible to use postcrosslinkers which comprise additionalpolymerizable ethylenically unsaturated groups, as described in DE 37 13601 A1.

The amount of postcrosslinker is preferably from 0.01 to 1% by weight,more preferably from 0.05 to 0.5% by weight, most preferably from 0.1 to0.2% by weight, based in each case on the polymer.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the postcrosslinkers.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium and strontium, trivalent cations such as the cationsof aluminum, iron, chromium, rare earths and manganese, tetravalentcations such as the cations of titanium and zirconium. Possiblecounterions are chloride, bromide, sulfate, hydrogensulfate, carbonate,hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogen-phosphate and carboxylate, such as acetate and lactate.Aluminum sulfate is preferred.

The amount used, based on the polymer particles, is, for example, from0.001 to 0.5% by weight, preferably from 0.005 to 0.2% by weight, morepreferably from 0.02 to 0.1% by weight.

The postcrosslinking is typically performed in such a way that asolution of the postcrosslinker is sprayed onto the hydrogel or the drypolymer particles. The spraying is followed by thermal drying, and thepostcrosslinking reaction can take place either before or during thedrying.

The spraying of a solution of the crosslinker is preferably performed inmixers with moving mixing tools, such as screw mixers, paddle mixers,disk mixers, plowshare mixers and shovel mixers. Particular preferenceis given to vertical mixers, very particular preference to plowsharemixers and shovel mixers. Suitable mixers are, for example, Lödigemixers, Bepex mixers, Nauta mixers, Processall mixers and Schugi mixers.

The thermal drying is preferably carried out in contact dryers, morepreferably shovel dryers, most preferably disk dryers. Suitable dryersare, for example, Bepex dryers and Nara dryers. Moreover, it is alsopossible to use fluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream dryer, for examplea staged dryer, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed dryer.

Preferred drying temperatures are in the range from 100 to 250° C.,preferably from 120 to 220° C. and more preferably from 140 to 200° C.The preferred residence time at this temperature in the reaction mixeror dryer is preferably at least 10 minutes, more preferably at least 20minutes, most preferably at least 30 minutes.

The process according to the invention enables the preparation ofwater-absorbing polymer particles with a high centrifuge retentioncapacity (CRC), a high absorption under a pressure of 4.83 kPa (AUP0.7psi) and high mechanical stability.

The water-absorbing polymer particles obtainable by the processaccording to the invention typically have the form of hollow spheres.The present invention therefore further provides water-absorbing polymerparticles comprising at least one cavity in the particle interior and atleast 1% by weight of water and/or at least one alkanolamine, thecrosslinking density at the particle surface having been increased.

The water content of the inventive polymer particles is preferably from1 to 30% by weight, more preferably from 2 to 20% by weight, mostpreferably from 3 to 15% by weight.

The inventive polymer particles comprise preferably from 0.5 to 30% byweight, more preferably from 1 to 20% by weight, most preferably from 2to 15% by weight, of at least one alkanolamine.

The inventive water-absorbing polymer particles are approximately round,i.e. the polymer particles have a mean sphericity (mSPHT) of typicallyat least 0.84, preferably at least 0.86, more preferably at least 0.88and most preferably at least 0.9. The sphericity (SPHT) is defined as

${{SPHT} = \frac{4\pi\; A}{U^{2}}},$where A is the cross-sectional area and U is the cross-sectionalcircumference of the polymer particles. The mean sphericity (mSPHT) isthe volume-average sphericity.

The mean sphericity (mSPHT) can be determined, for example, with theCamsizer® image analysis system (Retsch Technology GmbH; Germany).

Polymer particles with relatively low mean sphericity (mSPHT) areobtained by reverse suspension polymerization when the polymer particlesare agglomerated during or after the polymerization.

The water-absorbing polymer particles prepared by customary solutionpolymerization (gel polymerization) are ground and classified afterdrying to obtain irregular polymer particles. The mean sphericity(mSPHT) of these polymer particles is between approx. 0.72 and approx.0.78.

The present invention further provides water-absorbing polymer particlescomprising at least one cavity in the particle interior, thecrosslinking density at the particle surface having been increased, theparticles having a mean sphericity (mSPHT) of at least 0.84, preferablyof at least 0.86, more preferably of at least 0.88, most preferably ofat least 0.9, and the particles having an absorption index (AUP0.7 psiindex) of at least 60%, preferably of at least 70%, more preferably ofat least 80%, most preferably of at least 85%.

The present invention further provides water-absorbing polymer particlescomprising at least one cavity in the particle interior, thecrosslinking density at the particle surface having been increased,having a mean sphericity (mSPHT) of at least 0.84, preferably of atleast 0.86, more preferably of at least 0.88, most preferably of atleast 0.9, and a stability index of less than 0.15, preferably of lessthan 0.10, more preferably of less than 0.075, most preferably of lessthan 0.05.

The water content of the water-absorbing polymer particles is typicallyless than 10% by weight, preferably less than 8% by weight, morepreferably less than 6% by weight, most preferably less than 4% byweight, the water content being determined by the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP230.2-05 “Moisture content”.

The ratio of maximum diameter of the cavity to maximum diameter of thepolymer particle is preferably at least 0.1, more preferably at least0.3, most preferably at least 0.4.

The polymer particles comprise preferably at least 50 mol %, morepreferably at least 90 mol %, most preferably at least 95 mol %, ofpolymerized acrylic acid. The polymerized acrylic acid has preferablybeen neutralized to an extent of from 25 to 85 mol %, more preferably toan extent of from 50 to 80 mol %, most preferably to an extent of from60 to 75 mol %.

The present invention further provides water-absorbing polymer particleswhich are obtainable by the process according to the invention.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a centrifuge retention capacity (CRC) oftypically at least 20 g/g, preferably at least 25 g/g, preferentially atleast 30 g/g, more preferably at least 35 g/g, most preferably at least40 g/g. The centrifuge retention capacity (CRC) of the water-absorbingpolymer particles is typically less than 60 g/g.

The water-absorbing polymer particles obtainable by the processaccording to the invention have an absorption under a pressure of 4.83kPa (AUP0.7 psi) of typically at least 15 g/g, preferably at least 20g/g, more preferably of at least 25 g/g, most preferably of at least 30g/g. The absorption under a pressure of 4.83 kPa (AUP0.7 psi) of thewater-absorbing polymer particles is typically less than 50 g/g.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a permeability (SFC) of typically atleast 2×10⁻⁷ cm³s/g, preferably at least 10×10⁻⁷ cm³s/g, more preferablyat least 30×10⁻⁷ cm³s/g, even more preferably at least 60×10⁻⁷ cm³s/g,most preferably at least 200×10⁻⁷ cm³s/g. The permeability (SFC) of thewater-absorbing polymer particles is usually less than 500×10⁻⁷ cm³s/g.

The mean diameter of the polymer particles is preferably at least 200μm, more preferably from 250 to 600 μm, very particularly from 300 to500 μm, the particle diameter being determinable by light scattering andmeaning the volume-average mean diameter. 90% of the polymer particleshave a diameter of preferably from 100 to 800 μm, more preferably from150 to 700 μm, most preferably from 200 to 600 μm.

The present invention further provides processes for producing hygienearticles, especially diapers, comprising the use of water-absorbingpolymer particles produced by the abovementioned process.

The present invention further provides for the use of inventivewater-absorbing polymer particles in hygiene articles, for thickeningwastes, especially medical wastes, or as a water-retaining agent inmarket gardening.

The water-absorbing polymer particles are tested by means of the testmethods described below.

Methods:

The measurements should, unless stated otherwise, be performed at anambient temperature of 23±2° C. and a relative air humidity of 50±10%.The water-absorbing polymers are mixed thoroughly before themeasurement.

Mean Sphericity (mSPHT)

The mean sphericity (mSPHT) is determined with the Camsizer® imageanalysis system (Retsch Technology GmbH; Germany).

For the measurement, the product is introduced through a funnel andconveyed to the falling shaft with a metering channel. While theparticles fall past a light wall, they are recorded selectively by acamera. The images recorded are evaluated by the software in accordancewith the parameters selected.

To characterize the roundness, the parameter designated as sphericity inthe program is employed. The parameters reported are the meanvolume-weighted sphericities, the volume of the particles beingdetermined via the equivalent diameter xc_(min). To determine theequivalent diameter xc_(min), the longest chord diameter for a total of32 different spatial directions is measured in each case. The equivalentdiameter xc_(min) is the shortest of these 32 chord diameters. Theequivalent diameter xc_(min) corresponds to the mesh size of a screenthat the particle can just pass through. To record the particles, theso-called CCD-zoom camera (CAM-Z) is used. To control the meteringchannel, a surface coverage fraction of 0.5% is predefined.

Water Content

The water content of the water-absorbing polymer particles is determinedby the EDANA (European Disposables and Nonwovens Association)recommended test method No. WSP 230.2-05 “Moisture content”.

Centrifuge Retention Capacity (CRC Centrifuge Retention Capacity)

The centrifuge retention capacity of the water-absorbing polymerparticles is determined by the EDANA (European Disposables and NonwovensAssociation) recommended test method No. WSP 241.2-05 “Centrifugeretention capacity”.

Absorption Under Pressure (AUP0.7 psi Absorption Under Pressure)

The absorption under pressure is determined by the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP242.2-05 “Absorption under pressure”, using a weight of 49 g/cm² (0.7psi) instead of a weight of 21 g/cm² (0.3 psi).

Saline Flow Conductivity (SFC)

The saline flow conductivity of a swollen gel layer under a load of 0.3psi (2070 Pa) is, as described in EP 640 330 A1, determined as the gellayer permeability of a swollen gel layer of water-absorbing polymerparticles, except that the apparatus described on page 19 and in FIG. 8in the aforementioned patent application was modified to the effect thatthe glass frit (40) is no longer used, the plunger (39) consists of thesame polymer material as the cylinder (37) and now comprises 21 bores ofequal size distributed uniformly over the entire contact surface. Theprocedure and evaluation of the measurement remain unchanged from EP 640330 A1. The flow rate is recorded automatically.

The saline flow conductivity (SFC) is calculated as follows:SFC [cm³s/g]=(Fg(t=0)×L0)/(d×A×WP),where Fg(t=0) is the flow rate of NaCl solution in g/s, which isobtained by means of a linear regression analysis of the Fg(t) data ofthe flow determinations by extrapolation to t=0, L0 is the thickness ofthe gel layer in cm, d is the density of the NaCl solution in g/cm³, Ais the surface area of the gel layer in cm², and WP is the hydrostaticpressure over the gel layer in dyn/cm².Absorption Index (AUP0.7 psi Index)

The absorption index (AUP0.7 psi index) describes the mechanicalstability of the water-absorbing polymer particles.

To this end, 20 g of water-absorbing polymer particles are weighed intoa cylindrical porcelain mill with a capacity of approx. 360 ml. Theporcelain mill has an internal length of 8.8 cm and an internal diameterof 7.2 cm. In addition, 24 cylindrical porcelain bodies are introduced.The porcelain bodies have a height of 1.25 cm and a diameter of 1.25 cm.The weight of one porcelain body is 5.3 g. The cylindrical porcelainmill is closed and rolled by means of a roller-driven system at 150revolutions per minute for 15 minutes.

The absorption under pressure (AUP0.7 psi) of the water-absorbingpolymer particles is measured before and after mechanical stress.

The absorption index (AUP0.7 psi index) is calculated as follows:AUP0.7 psi index=AUP0.7 psi_(after)/AUP0.7 psi_(before)×100%where AUP0.7 psi_(before) is the absorption under pressure (AUP0.7 psi)of the polymer particles before the mechanical stress and AUP0.7psi_(after) is the absorption under pressure (AUP0.7 psi) of the polymerparticles after the mechanical stress.Stability Index

The stability index describes the mechanical stability of thewater-absorbing polymer particles.

The mechanical stress is carried out as already described for theabsorption index.

The proportion of water-absorbing polymer particles having a particlesize of less than 100 μm is measured before and after the mechanicalstress. When the water-absorbing polymer particles comprise more than 1%by weight of particles having a particle size of less than 100 μm, theseshould be removed beforehand.

The stability index is calculated as follows:Stability index=(particles<100 μm_(after)−particles<100μm_(before))/100% by wt.where particles<100 μm_(before) is the proportion by weight of polymerparticles having a particle size of less than 100 μm before themechanical stress and particles<100 μm_(after) is the proportion byweight of polymer particles having a particle size of less than 100 μmafter the mechanical stress.

The particles having a particle size of less than 100 μm are determinedphotooptically with a PartAn 2001 F/L particle analyzer (from AnaTec,Duisburg, Germany). For the measurement, 20 g of polymer particles areused.

EXAMPLES Example 1

14.3 kg of sodium acrylate (37.5% by weight solution in water) and 1.6kg of acrylic acid were mixed with 29 g of 15-tuply ethoxylatedtrimethylolpropane triacrylate. The solution was dropletized into aheated dropletization tower filled with a nitrogen atmosphere (180° C.,height 12 m, width 2 m, gas velocity 0.1 m/s in cocurrent). The meteringrate was 16 kg/h. The dropletizer plate had 30×170 μm bores. Thediameter of the dropletizer plate was 65 mm. The initiator was mixedwith the monomer solution just upstream of the dropletizer by means of astatic mixer. The initiator used was a 6.5% by weight solution of2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride in water. Themetering rate of the initiator solution was 0.44 kg/h. The gas outlettemperature from the dropletization tower was 126° C.

Subsequently, the resulting water-absorbing polymer particles werepostcrosslinked. To this end, 200 g of water-absorbing polymer particleswere sprayed with 6.3 g of postcrosslinker solution by means of atwo-substance nozzle in a food processor at a medium stirrer level. Thepostcrosslinker solution consisted of 0.3 g of2-hydroxyethyl-2-oxazolidone, 4.2 g of water and 1.8 g of isopropanol.The moist polymer was homogenized once again with a spatula andheat-treated at 170° C. in a forced-air drying cabinet for 90 minutes.The postcrosslinked polymer particles were freed of lumps by means of an850 μm screen.

Example 2

The postcrosslinked water-absorbing polymer particles obtained accordingto Example 1 were coated. To this end, 200 g of postcrosslinkedwater-absorbing polymer particles were preheated at 80° C. in a heatedcabinet for at least 30 minutes and sprayed with the coating agent bymeans of a two-substance nozzle in a food processor at a medium stirrerlevel. The coating agent used was 8.0 g of water. Stirring was continuedfor one minute. After storage for 16 hours, the coated polymer particleswere freed of lumps by means of an 850 μm screen.

The results are summarized in Table 1.

Example 3

The procedure of Example 2 was repeated. The coating agent used was asolution of 8.0 g of water and 0.306 g of sorbitan monolaurate.

The results are summarized in Table 1.

Example 4

The procedure of Example 2 was repeated. The coating agent used was asolution of 8.0 g of water, 0.306 g of sorbitan monolaurate and 3.0 g oftriethanolamine. The results are summarized in Table 1.

Example 5

The procedure of Example 2 was repeated. The coating agent used was asolution of 8.0 g of water, 0.306 g of sorbitan monolaurate and 6.0 g oftriethanolamine.

The results are summarized in Table 1.

TABLE 1 Analysis results AUP 0.7 psi AUP 0.7 psi AUP 0.7 psi Particles <100 μm Particles < 100 μm Stability Ex. (before) (after) index (before)(after) index 1*) 24.7 g/g 12.8 g/g 51.8% 0.2% by weight 30.9% by weight0.31 2 24.2 g/g 16.4 g/g 67.9% 0.1% by weight 20.3% by weight 0.20 324.1 g/g 19.0 g/g 78.8% 0.1% by weight  6.6% by weight 0.065 4 23.6 g/g18.4 g/g 78.0% 0.1% by weight  6.6% by weight 0.065 5 22.8 g/g 19.4 g/g85.1% 0.1% by weight  4.8% by weight 0.047 *)comparative example

1. A process for producing water-absorbing polymer particles comprising(i) polymerizing droplets of a monomer solution comprising a) at leastone ethylenically unsaturated monomer, b) optionally at least onecrosslinker, c) at least one initiator, d) water, in a gas phasesurrounding the droplets, (ii) the resulting polymer particles beingpostcrosslinked, then (iii) at least partly coating the postcrosslinkedpolymer particles with water or an aqueous solution comprising sorbitanmonoester and/or an alkanolamine.
 2. The process according to claim 1,wherein a solids content of the monomer solution is at least 35% byweight.
 3. The process according to claim 1, wherein the monomer a) isat least partly neutralized acrylic acid to an extent of at least 50 mol%.
 4. The process according to claim 1, wherein the droplets have a meandiameter of at least 100 μm.
 5. Water-absorbing polymer particlesproduced according to claim
 1. 6. Water-absorbing polymer particlesaccording to claim 5 comprising at least one cavity in the particleinterior and at least 1% by weight of water and/or at least onealkanolamine, a crosslinking density at the particle surface having beenincreased.
 7. The polymer particles according to claim 5, wherein thepolymer particles have a mean sphericity of at least 0.84. 8.Water-absorbing polymer particles according to claim 5 comprising atleast one cavity in the particle interior, a crosslinking density at theparticle surface having been increased, and the particles having a meansphericity of at least 0.84 and an absorption index of at least 60%. 9.Water-absorbing polymer particles according to claim 5 comprising atleast one cavity in the particle interior, a crosslinking density at theparticle surface having been increased, and the particles having a meansphericity of at least 0.84 and a stability index of less than 0.15. 10.The polymer particles according to claim 5, which have at least onecavity and wherein a ratio of maximum diameter of the cavity to maximumdiameter of the polymer particle is at least 0.1.
 11. The polymerparticles according to claim 5, which comprise at least partiallyneutralized polymerized acrylic acid to an extent of at least 50 mol %.12. The polymer particles according to claim 5, wherein the polymerparticles have an absorption under pressure (AUP0.7 psi) of at least 15g/g.
 13. The polymer particles according to claim 5, which have a meandiameter of at least 200 μm.
 14. A hygiene article comprising polymerparticles according to claim
 5. 15. The polymer particles according toclaim 6, wherein the polymer particles have a mean sphericity of atleast 0.84.